Orthopedic implants are used to strengthen or replace joints that typically experience high levels of stress and wear. In a primary replacement surgery, joints that have experienced trauma or have been worn to a degree that inhibits normal functioning of the joint are replaced or reinforced with stronger wear-resistant implants. In a revision surgery, primary implants that have either been unsuccessful or have worn to a degree that inhibits their function are supplemented or replaced with revision implants. The constant daily stress and wear at the replacement joints, especially for weight-bearing knee replacements, require both primary and revision implants to be strong enough to withstand significant abuse.
Conventional knee implants are made of rigid, high-modulus metals used to provide ample support and to withstand the high stresses typically present at the knee joint. High-strength metals such as titanium, stainless steel, zirconium, cobalt-chrome alloy, and other metal alloys are often used in femoral and tibial implants to achieve the strength and stability required. These metal components exhibit higher rigidity and higher modulii of elasticity than the bone structure into which they are implanted in order to withstand the significant stresses that are incident on the knee joint during everyday activity. The size of the implants and the metal materials used to make them results in a heavy implanted component that a patient must adjust to, and patients may experience discomfort from “feeling the weight” of the metal component after implantation.
With the rigid materials and the large implant designs of traditional knee implant components, the knee replacements bear a majority of stresses incident on a patient's joint after implantation. As a result, the bone surrounding the implant often experiences lower stresses than normal. The decreased stresses cause bone resorption as the bone breaks down to adjust and accommodate the decreased need for support from the bone at the joint. In response to the changes in normal loading, a bone remodels itself to either build up more mass to strengthen the bone or break down bone mass to weaken the bone. This process is known as Wolff's law, and it causes a normal bone to become stronger if loading on the bone increases or weaker if loading on the bone decreases.
Bone resorption around an implant can have significant negative effects, as it decreases the integrity of the bone and its ability to hold the implant solidly in place. This complication is known as stress shielding as the high-strength implant “shields” the surrounding bone from stress and loading that is necessary to cause the bone to maintain its strength. The resulting bone resorption causes patients to experience pain and feel the weight of their knee implants during their everyday activities. In some cases, the bone resorption resulting from stress shielding can even cause an implant to fail completely, necessitating a second revision knee replacement surgery.